Introduction

Pain is a personal, complex experience influenced by biological, psychological, and social factors. The International Association for the Study of Pain (IASP) defines pain as “an unpleasant sensory and emotional experience associated with … actual or potential tissue damage” acknowledging that animals learn concepts, including that of pain, and the inability to communicate through language does not negate the possibility that an animal is experiencing pain (Raja et al. 2020).

The sensory component of pain, resulting from activity in specialized neurons, refers to “what it feels like” (i.e. perceptual qualities such as mild, severe, burning, tingling, etc.). The emotional component of pain refers to “how it makes one feel” (i.e. fearful, frustrated, anxious). Pain burden refers to the negative effects of pain for the animal and the owner/caregiver. For the animal, pain has a negative impact on physical health and function, nutrition, behavior, socialization, and mental state (Steagall et al. 2021). Pain has negative effects on production (i.e. animals raised for human consumption may have decreased food intake, impaired growth, and reproduction) and quality of research studies (i.e. pain in laboratory animals may produce erroneous results). The pet–owner relationship may be taxed or broken (i.e. pets with chronic pain require significant financial, time, and physical commitments from owners, and caring for them can be emotionally demanding; Spitznagel et al. 2018). An overlooked issue is the impact that painful animals can have on veterinary team members, including emotional distress and compassion fatigue.

Understanding the Difference Between Nociception and Pain

Pain and nociception are different. Nociception refers to the neural processing of noxious stimuli that could or does cause tissue damage (Sneddon 2018). It occurs without conscious perception and can result in reflex withdrawal. The “pain pathway” comprises four main steps. The first step is transduction – activation of peripheral nociceptors located in the skin, muscles, joints, and viscera, by noxious or potentially damaging stimuli. Such stimuli cause depolarization of nerve cell membranes with generation of an action potential. Step 2 is transmission from the periphery to the dorsal horn of the spinal cord in the central nervous system. The third step is modulation – the nociceptive signal can be amplified (pain facilitation) or decreased (pain inhibition) by various mechanisms. Thus, nociception comprises steps 1 to 3. The fourth step is perception – the nociceptive signal is integrated in the cerebral cortex and emotional attributes are incorporated with sensory signals (i.e. pain is consciously perceived) (Klinck and Troncy 2016). It is only when the nociceptive signals reach the conscious brain that it can be called “pain.” For example, in an anesthetized patient, nociception is occurring, but there is no conscious experience of pain. Once the patient recovers from anesthesia and regains consciousness, the nociceptive stimuli (e.g. peripheral and/or central sensitization) will be perceived. That is why pain must be prevented and treated when patients are under general anesthesia and painful procedures are being performed.

Types of Pain

Adaptive or acute pain refers to pain that is protective, either to protect a healthy area from injury or a damaged area from further injury. The severity of adaptive pain correlates with the degree of injury (e.g. minor or major surgery, trauma, wounds, burns, intestinal obstruction, and pancreatitis). Although usually self-limiting, adaptive pain should be treated. Maladaptive or chronic pain refers to pathological pain that serves no biological function. Maladaptive pain does not always correlate with the inciting lesion or disease severity (e.g. osteoarthritis, invasive tumors, periodontal disease). Maladaptive pain can develop from adaptive pain (e.g. due to inadequate perioperative pain management or in conditions with extensive damage to soft tissues, bone, and neural tissues) resulting in persistent postsurgical pain (e.g. following amputations including tail docking and onychectomy). Finally, maladaptive pain can occur without any identifiable primary cause (i.e. maladaptive pain is a disease in its own right).

Pain can be categorized in different ways including:

• Nociceptive pain: protects from self-harm. Examples: contact with a hot object (thermal nociception), pressure against a sharp surface (mechanical nociception).
  
• Inflammatory pain: occurs in adaptive or maladaptive pain when there is a tissue lesion, release of inflammatory cytokines, and sensitization of sensory nerves. Examples: postoperative pain, trauma, osteoarthritis, chronic periodontal disease, wounds, and otitis.
  
• Neuropathic pain occurs when there is a lesion or disease of the somatosensory system: it may be peripheral and/or central. Examples: intervertebral disc disease, nerve sheath tumors.
  
• Cancer pain: arises from cancers resulting in the release of chemicals that contribute to cancer growth and the generation and maintenance of pain. Additional pain may be induced by treatments, e.g. radiation therapy or surgery.
  
• (Dys)functional pain: disorders for which no organic disease can be identified despite comprehensive investigation, i.e. feline idiopathic cystitis.

Pain can be spontaneous due to natural disease or induced by humans caring for animals or by veterinary health professionals during medical interventions. Examples of spontaneous pain include acute pancreatitis, degenerative diseases (e.g. osteoarthritis), stomatitis, and wounds. Examples of induced pain include pain due to inappropriate handling or equipment (e.g. a harness that causes chronic wounds), inadequate housing conditions (e.g. restricted movement causes musculoskeletal pain; inappropriate flooring contributes to lameness), and genetic selection (e.g. broiler chickens selected for fast growth suffer from lameness, mobility impairment, and even fractures because their skeletal growth does not keep pace with their muscle growth) (Nalon and Stevenson 2019). Examples of induced pain caused by medical interventions include procedures performed in laboratory animals for the study of pain (e.g. nerve constriction for induction of neuropathic pain, transection of the cranial cruciate ligament for induction of osteoarthritis), and common veterinary procedures such as neutering surgeries, wound management, and diagnostic procedures.

Pain and Emotions

Pain may have far-reaching and long-term consequences. The pain experience is modulated by the neurobiology of the individual, past experiences, environmental and social factors, and emotions. There is a complex bidirectional interrelationship between pain and emotions.

Long-term pain negatively impacts cognitive functioning and produces anxiety-like behaviors weeks to months after initial injury (Bushnell et al. 2015). Pain causes negative emotions, whereas emotions modulate the pain experience. Emotions from higher brain centers modulate nociceptive signals in the spinal cord and can decrease or increase this signal before it reaches the brain where it is perceived. Overall, positive emotions decrease pain perception and negative emotions increase pain perception (Finan and Garland 2015; Hanssen et al. 2017). Therefore, if we promote positive experiences, we can refocus the patient’s attention to pleasurable and rewarding experiences helping them build emotional resilience and coping abilities (Finan and Garland 2015; Mills et al. 2020). Optimizing the animal’s environment to provide opportunities for performance of species-specific motivated behaviors (e.g. rewarding experiences such as play, appropriate socialization, and exploratory or predatory behaviors) can decrease the pain experience (Bushnell et al. 2015).

Not only is pain unpleasant to the individual experiencing it, pain in conspecifics is also unpleasant and laboratory studies show emotional contagion and social modulation of pain. For example, pain behaviors increase when cage mate mice are exposed to a painful stimulus together, and pain behaviors decrease when nonpainful mice spend more time near a painful cage mate (Langford et al. 2006, 2010; Mogil 2019).

Another interesting relationship between pain and emotions is that fear and stress can cause painful conditions. A classic example is idiopathic cystitis in domestic cats, which is associated with poor emotional health due to potential threats to their perception of control (Buffington and Bain 2020).

The Debate of the Pain Experience in Animals

The neurobiology of pain is similar among mammalian species. Some of these similarities include anatomy, peripheral nociceptors and their electrophysiological properties, mechanisms of pain modulation, and peripheral and central sensitization (Sneddon 2018). The clinical symptomatology and response to therapy of conditions affecting humans and other mammals are also similar. Indeed, animals have been crucial for our understanding of pain as attested by their widespread use in translational pain research (Mogil 2019). For example, osteoarthritis is biomechanically, histologically, and molecularly similar between dogs and people (McCoy 2015). Dogs and cats with spontaneous osteoarthritis develop peripheral and central sensitization in addition to negative effects on their quality of life including decreased physical activity and sleep disturbances, which are also reported in humans with osteoarthritis (Knazovicky et al. 2016; Monteiro et al. 2020). Osteosarcoma, a very painful bone cancer, is biologically similar in children and dogs (Simpson et al. 2017).

The debate as to whether invertebrates and nonmammalian vertebrates experience pain (and not only nociception) is ongoing: yet scientific evidence from physiological (Sneddon 2018) and behavioral observations strongly suggest an affective pain experience (Baker et al. 2019; Crook 2021). For example, animals display species-specific changes in behavior related to pain (e.g. grooming in octopuses, tail beating in zebrafish) and are motivated to avoid locations previously associated with pain, which indicates they learn from a painful experience, one of the key points in the IASP’s definition of pain (Raja et al. 2020). Pain-related behaviors are not seen in the absence of painful stimuli and resolve when analgesics are given. Together, these observations indicate that these animals experience negative emotions associated with pain (i.e. the experience is not only sensorial: a negative emotion is attached to it that affects their present and future behaviors to avoid the experience).

From an evolutionary perspective, pain most likely did not appear de novo in humans: the functions and mechanisms of pain are products of prior evolution (Walters and Williams 2019). However, the clinically oriented definition of pain in humans has profound legal and ethical implications in protecting animals because producing compelling evidence of conscious pain perception in nonhuman animals is a complicated task (Walters and Williams 2019). This is a topic of debate among scientists studying animal behavior while aiming to avoid anthropomorphism and skeptics insisting on anthropodenial. Differences in neurobiology among species such as humans and rodents are of quantity and not quality (Mogil 2019) and complex social/cognitive phenomena occur similarly between such species.

If we describe sentience as “feelings that matter” (positive and negative) and state that the prerequisites for suffering are sentience and consciousness, our task is to define how sentience is experienced in animals of different taxa and under different circumstances. Indeed, animal sentience and capacity to suffer are the basis of animal welfare science and available frameworks for welfare assessment (UK Farm Animal Welfare Council 1979; Mellor et al. 2020). Comparative and translational pain research is focused on similarities among different species, particularly with regard to the affective component of pain, and is likely to contribute substantially to ethical discussions related to pain in animals and their consequent legal protection. When we look at the many shortcomings of current animal welfare legislation it is sobering to reflect that it was in 1789 that the British philosopher Jeremy Bentham stated (referring to animals) “The question is not can they reason? Nor can they talk? But can they suffer?” (Bentham 2000). Based on such an expansive body of evidence, it is reasonable to assume that what is painful for humans is painful for animals.

Laws, Regulations, Policies, and Guidelines Pertaining to Pain Management in Domestic Animals

Ethical Duty of Veterinary Professionals to Manage Pain

Veterinarians have an ethical and medical duty to prevent, diagnose, and treat pain. The ethical responsibilities are stated in the AVMA Veterinarian’s Oath. Although the word “pain” does not specifically appear, it is implied within the concept of “suffering”: “prevention and relief of animal suffering” (AVMA n.d.c). Furthermore, the AVMA Principles of Veterinary Medical Ethics state that the practice of veterinary medicine requires one to “diagnose, prognose, treat, correct, change, alleviate, or prevent … pain” (AVMA n.d.d). Similarly, the Veterinary Technician Code of Ethics states that professionals “shall prevent and relieve the suffering of animals with competence and compassion” (NAVTA 2007). Violation of the AVMA Principles of Veterinary Medical Ethics might result in disciplinary action or legal prosecutions: however, this rarely happens when substandard pain management is identified since punishment requires substantial proof of misconduct, animal cruelty, or neglect. The AVMA has numerous policies related to pain management in several species. Voluntary adherence to such policies is encouraged but does not supersede laws or regulations.

The medical obligations refer to the fact that pain has negative consequences including activation of the sympathetic nervous system, increased secretion of stress hormones, immunosuppression, decreased food intake, altered function (e.g. lameness), impaired healing, increased morbidity, etc. (Steagall and Monteiro 2019; Steagall et al. 2021).

It is curious that despite these ethical and medical duties of veterinarians, there is a general acceptance of painful procedures in farm animals without mitigation of pain (i.e. speciesism). A classic example is the castration of piglets without anesthesia/analgesia (Yun et al. 2019). One could argue that in such circumstances, a veterinarian is disrespecting the oath and their medical responsibilities (i.e. pain is not being prevented or treated). Nevertheless, the issue is not that simple: it is one of cultural and societal norms, and the triad relationships between the animal, veterinarian, and client. A veterinarian may well face an ethical dilemma if the farmer refuses to pay for effective anesthetics/analgesics for piglet castration (Scollo et al. 2021) (i.e. protecting the animal versus complying with the client’s request). This can be approached from a principlist theory based on respect for autonomy, nonmaleficence, beneficence, and justice (Beauchamp 2016). In this example, the veterinarian must: respect a client’s autonomy; abstain from causing harm; act for another’s benefit (to prevent harm and heal when harm has occurred); and consider the moral rules of justice according to what is fair, due, or owed. Based on the principlist theory, the Veterinarian’s Oath, and the Principles of Veterinary Medical Ethics, prevention of animal suffering should, in this example, prevail over the client’s wishes, but often does not.

Ethical conflicts are part of everyday veterinary practice and different stakeholders must be considered including but not limited to the animal, owner/caretaker, attending veterinarian, policy makers, and the public. Using the same example of castration in piglets, we might analyze the situation using different ethical theories (see Chapter 4). From a contractarian view, a person’s own interest is what matters most. For a contractarian, piglets are not moral subjects and are not worthy of moral concern including treatment of pain (Gjerris et al. 2013). From an animal rights view, there are things that you simply cannot do to animals. Animals, like people, have rights. Rights are moral constraints (“subject-of-a-life”), and it is simply wrong to violate one’s rights, which includes castration. A utilitarian view is based on maximizing welfare for the largest number of individuals, even if it means that some will suffer to benefit the most. This approach is often referred to as “the end justifies the means.” The impact of choices on the welfare of all concerned parties is taken into consideration. While the goal is to choose the course of action that will result in the largest total sum of welfare, in most cases, the interests of humans supersede those of animals (Gjerris et al. 2013). Despite different ethical views, the Veterinarian’s Oath should prevail and guide decision making with a focus on preventing animal suffering and protecting animal welfare above all else.

Other examples of common ethical issues in veterinary pain medicine include: onychectomy in cats (amputation of the last digits of the front and/or hind limbs); “cosmetic” or “convenience” surgeries in dogs (ear cropping and caudectomy); devocalization in dogs; debeaking in chickens; and castration, tail docking, dehorning/disbudding, branding, ear notching/tagging, and nose ringing in pigs, cattle, and/or sheep. In farm animals, painful procedures are generally performed for human safety (e.g. dehorning), identification (e.g. branding, tagging), or preventing problems usually caused by inadequate housing and/or environmental conditions (e.g. tail biting, feather pecking). The ethical issues in these examples are twofold: one because these procedures are not usually medically justified; and two because frequently pain management is either omitted or inadequate. Extensive literature demonstrates that these animals display pain behaviors during and after these procedures and that such behaviors subside with multimodal pain management (Steagall et al. 2021).

Measures to avoid painful procedures in farm animals already exist or are being studied and include but are not limited to: genetic selection of cattle without horns; production of pigs without castration (slaughter at a younger age before boar taint occurs); improving housing, handling, and environmental enrichment of animals to improve welfare and remove the need for tail docking or debeaking; and attaching sensors or collars for animal identification to avoid branding or tagging. In companion animals, there is a need to change breed standards to avoid ear cropping and tail docking in dogs and to promote owner education regarding feline behavior to avoid onychectomy.

Assessing and Treating Pain in Animals

The first steps in treating pain are to anticipate it, look for it, recognize it, and in some way quantify it. Methods of pain assessment range from the highly subjective (i.e. opinion) to sophisticated and objective analysis of activity, body posture, and facial expressions. Pain assessment in animals can be performed using different methods such as observing behavior, quantitative sensory testing, biomarkers, etc. Although physiological parameters such as heart and respiratory rates or levels of cortisol have been used to assess pain, these are nonspecific, are influenced by fear and anxiety, and should not be relied upon for the evaluation of pain (Quimby et al. 2017). In practice, assessment usually relies on the observation of pain-related behaviors.

An important aspect of pain assessment is to evaluate the behaviors of animals before a painful insult such as surgery. Pain assessment can be particularly challenging in animals with shy or fearful temperaments: therefore, knowing the normal behavior of an animal before surgery can help with interpretation of pain-related behaviors following surgery (Steagall and Monteiro 2019). The most common abnormal behaviors observed in animals with acute pain are noted in Table 19.1. Chronic pain has been less extensively studied and available data generally relate to musculoskeletal conditions. Common abnormal behaviors related to chronic pain are listed in Table 19.2.

Pain assessment should utilize pain scales. Pain scales are noninvasive, inexpensive, do not require any equipment or restraint, and may be performed by remote observations (Luna et al. 2020). For this to be performed systematically and with confidence, robust species-specific scientific validation must be performed to ensure that pain scales measure what they are designed to measure (i.e. pain and not sedation related to anesthetic drugs), are repeatable with different observers and within the same observer across time, and that clinically relevant changes are detectable such as differences in pain scores between painful and pain-free animals or before and after analgesic administration (Steagall and Monteiro 2019). The development and validation of species-specific pain scales is a relatively recent area of research and only a few instruments have undergone validation. To complicate things further, pain-related behaviors are different in acute and chronic painful conditions: thus, pain scales should target these differences. For a comprehensive assessment of available facial grimace scales in nonhuman mammals and their level of scientific validation, see Evangelista et al. 2021. A recent systematic review of pain scales in farm animals is also available (Tomacheuski et al. 2021). Finally, veterinary technicians/nurses can greatly contribute to pain assessment and might be the primary detectors of pain in practice because they often spend more time with patients than veterinarians. They should be involved in this task and trained in using pain scales.

Just as important as using validated pain scales for pain assessment is knowledge of pharmacological and nonpharmacological modalities of pain management. Pain should be anticipated and prevented in addition to being treated. For these reasons, veterinary professionals need to be familiar with the concepts of pre-emptive, preventive, and multimodal analgesia (Mathews et al. 2014). Pre-emptive analgesia involves the administration of analgesic drugs prior to a painful insult such as surgery to reduce or prevent nociceptive impulse transmission within the pain pathway. This approach is more effective in reducing pain than using a similar analgesic protocol after the painful insult. However, in most cases, analgesic treatment must continue during and after surgery; the duration of postsurgical treatment should be related to the severity of tissue injury and pain assessments. Using analgesics for the appropriate duration is termed preventive analgesia. Multimodal analgesia refers to the use of different classes of analgesics acting at different locations in the pain pathway. This results in synergism among analgesics, improved pain control, reduced doses of each drug, and fewer adverse effects. One example may include use of local anesthetic, opioids, and sedative/anxiolytics.

A holistic approach toward pain management must be undertaken and factors affecting patient comfort should be considered. For example, thermal and acoustic comfort, fear-free and low-stress handling techniques, prevention and control of nausea and vomiting, using harnesses in mobility-impaired patients, ensuring opportunities to void, emptying the urinary bladder, etc. Particularly in companion animal practice, these considerations reinforce the concept of care-based ethics as veterinary professionals are the ones caring for those at risk of suffering. Such relationships (between the responsible and the vulnerable) might be viewed as a therapeutic alliance for optimal pain management (Carvalho et al. 2018). Patient-centered practice is increasingly important with the evolving status of animals in modern societies (Grimm et al. 2018).

Decisions around treatment of pain depend on the triad of owner, patient, and clinician (See Case Studies 19.1 and 19.2). For example, balancing clients’ expectations of having their pet home soon after surgery and minimizing costs are weighed against the potential stress to the animal of a longer hospital stay and higher costs, but where monitoring and tailored analgesic administration can occur. Following major surgery, many animals should remain hospitalized for monitoring and continuous administration of analgesics. If the hospital where the surgery was performed cannot provide 24/7 coverage, the patient should be referred to another facility where appropriate care can be provided. The availability of long-acting drugs approved for use in dogs and cats such as liposome-encapsulated bupivacaine, which has a duration of up to 72 hours, and special formulations of long-acting opioids (e.g. buprenorphine for cats) allows some patients to be returned home earlier with the knowledge that analgesics should still be effective.